1. Field of the Invention
The present invention relates to a semiconductor physical quantity sensor, having a beam-structure movable portion and a fixed portion, for measuring a physical quantity, such as acceleration, yaw rate or vibration by detecting a capacitance change between the beam-structure movable portion and the fixed portion, and a method of producing such a sensor.
2. Description of Related Art
A conventional semiconductor physical quantity sensor having a beam-structure movable portion is a known differential servo-control capacitance acceleration sensor employing an attached substrate unit (as disclosed in Japanese Patent Application Laid-open No. Hei. 9-211022).
This sensor, formed from a beam-structure movable portion and a fixed portion on a substrate (support substrate), detects a physical quantity by detecting a capacitance change between the beam-structure movable portion and the fixed portion. The beam-structure has a first anchor and a mass portion supported by the anchor via a beam portion. This mass portion has a movable electrode that moves upon application of a force caused by acceleration. The fixed portion has a fixed electrode facing the movable electrode and fixed to the substrate by a second anchor. Further, the substrate has an attaching thin film formed on a semiconductor substrate, an insulating film formed on the attaching thin film and a conductive film formed on the insulating film. The first and second anchor portions are formed from a conductive film.
In this acceleration sensor, sensitivity of the sensor is largely affected by parasitic capacitance formed between the conductive film and the insulating film or the attaching thin film. Specifically, when a capacitance between the beam-structure and fixed portion is measured, sensor output is represented by (capacitance variation)/((total capacitance)+(parasitic capacitance)). Therefore, when the attaching film is electrically floated, the parasitic capacitance is, and the sensitivity of the sensor is small.
In general, the above semiconductor physical quantity sensor is produced by employing a semiconductor producing method such as etching, so that a separation trench separating the movable portion from the fixed portion is formed in an element forming film formed on the support substrate. Therefore, at a surrounding portion of a sensor element portion having the movable portion and the fixed portion, there is an element forming film portion that is another portion of the sensor element portion.
Furthermore, although the surrounding portion is supported by the support substrate, this portion is electrically floated. This structure leads to change the sensor output as in the attaching thin film structure.